1. Field of the Invention
The present invention relates to a mass spectrometer for measuring the mass of a gas molecule in a reduced-pressure (vacuum) atmosphere. More particularly, the present invention relates to a mass spectrometer which can be used in a relatively high pressure atmosphere of 0.1 Pa or more, a small-size mass spectrometer capable of measuring a high-mass molecule at a high sensitivity, and a mass spectrometer capable of measuring an ultra fine amount of gas.
2. Prior Art
A Q-pole type mass spectrometer, called mass filter or quadrupole type mass analyzer, is capable of carrying out high-sensitivity measurement in a wide dynamic range with a small and simple structure under easy control. Therefore, the Q-pole type mass spectrometer is a general mass spectrometer for measuring the mass of a gas molecule.
The Q-pole type mass spectrometer is comprised of an ion source for ionizing gas, a Q-pole for carrying out mass separation and a collector for detecting mass-separated ions. The Q-pole type mass spectrometer is actuated in a low pressure atmosphere of 0.01 Pa or less.
FIG. 9 shows a conventional Q-pole type mass spectrometer under ordinary operating condition.
Four Q-poles 1 (poles) are disposed in parallel at a high precision of micro order, and ordinarily the length is 100 to 300 mm while an interval between opposing poles is 5 to 10 mm. A high-frequency voltage V of 1 to 5 MHz and DC voltage U are applied to each pole. Accurately speaking, the same V, U voltages are applied to opposing poles and −V, −U voltages are applied to neighboring poles. Consequently, a specific quadrupole electric field (bipolar electric field) is formed in the diameter direction.
Ions existing near the axis of this quadrupole electric field are vibrated in the diameter direction by Coulomb force and ions except for those of a mass and electric charge determined by the V, U values are expelled out of the axis.
On the other hand, with respect to the potential in the axial direction, the potential is the same at any axial point so that there is no electric field (rate of potential position change) in the axial direction. Thus, no Coulomb force is generated on ion in the axial direction. The reason why the same potential is produced at any axial point is that the four poles are united, the united pole has the same potential, and the voltage does not change depending on any position in the axial direction of the pole. Thus, the same electric field is formed in a section vertical to the axis in any axial direction so that no electric field is generated in the axial direction.
Usually, the voltage of the ion source is raised above the potential of the Q-pole on the axis (center potential of the quadrupole electric field) by about 10 V and then the ion is advanced in the Q-pole at a speed (10 eV) corresponding to linear energy of 10 eV. At this time, with respect to the diameter direction, only an ion having a specific mass/charge continues to vibrate stably. Then only a specific ion passes the Q-pole so that it is detected by the collector and becomes a signal. The other ions which do not have the specific mass/charge are expelled halfway. Thus, the motion of ions in the diameter direction and the motion of ions in the axial direction are completely independent of each other in the Q-pole.
By changing the ratio between V and U, the mass/charge of the ion, which is to be measured, can be selected and an ion of about 1 to 1000 amu (atomic mass unit) can be measured. However, to separate the mass of an ion with mass number M amu with sufficient resolution, the ion needs to be vibrated at least 2 to 4 times (M/0.5)0.5 in the Q-pole. That is, it needs to be vibrated 5 times at 2 amu, 30 times at 50 amu, 50 times at 100 amu and about 100 times at 300 amu.
Therefore, it is necessary that the time within which the ion to be measured passes through the Q-pole is longer than time required for this vibration.
The ion speed allowing mass separation to be achieved is determined by a relation between the length of the Q-pole and the high-frequency vibration number. For example, if the length of the Q-pole is 200 mm and the high-frequency vibration number is 2 MHz, the necessary vibration number in all mass ranges is satisfied at a speed of 15 eV. Therefore, the speed of an ion capable of achieving mass separation is about 15 eV max. and a speed of 5 to 10 eV is necessary to obtain sufficient resolution.
The Q-pole type mass spectrometer is used in an atmosphere of 0.01 Pa or less. If it is operated in a high-pressure atmosphere of 0.01 Pa or more, collisions between the atmospheric gas and the ion occurs so as to obstruct proper measurement. This will be described below.
The mean free path is an average distance in which an ion or the like can advance without any collision with the atmospheric gas. And the mean free path is in inverse proportion to the pressure (density) of the atmosphere. In a strict sense, the mean free path relates to the atmospheric gas, and the size, mass and speed of the ion, so that the mean free path depends on not only the pressure but also the kind of gas and ion speed. In an Ar (Argon) atmosphere of 0.1 Pa, the mean free path is about 120 mm for a He ion (4 amu), about 60 mm for a CO2 ion (44 amu) and about 33 mm for a large ion of 300 amu.
If the mean free path of the ion is smaller than the length of the Q-pole, for example, and the atmospheric pressure is 1 Pa, the ion passing through the Q-pole always collides with the atmospheric gas, statistically speaking (on average). For simplification, it is assumed that the collision occurs front to front in the axial direction (although there are actually collision components in the diameter direction, they are offset by each other on average so that they can be omitted).
If the mass of an ion is larger than that of the atmospheric gas, the ion receives the pressure of the atmospheric gas upon collision so that the ion speed is largely reduced. Therefore, the ion speed in the axial direction drops each time a collision occurs and finally the ion is stopped in the Q-pole. However, there is no change in the vibration in the diameter direction. FIG. 10 shows this condition.
The deceleration rate decreases as the ratio of mass between ion and the atmospheric gas increases. That is, a heavy ion is not decelerated as much. On the other hand, if the mass of an ion is smaller than the atmospheric gas, the ion is repelled after a collision so that the ion advance direction is inverted. If the masses of an ion and the atmospheric gas are the same, the ion is stopped with a single collision. The change of the speed between before and after a collision is expressed by the following equation.V2=V1(Mi−Mg)/(Mi+Mg)where, V1: ion speed before collision, V2: ion speed after collision, Mi: mass of ion, Mg: mass of the atmospheric gas.
Anyway, deceleration including stop and retraction is generated by a collision with the atmospheric gas so that advance of an ion in the Q-pole is hampered. Thus, usually, the Q-pole type mass spectrometer is used under a pressure of 0.01 Pa or less in which the mean free path is longer than the length of the Q-pole.
Thus, for measurement of gas at a pressure of more than 0.01 Pa, it is requested to reduce the pressure in the region of the Q-pole type mass spectrometer by differential air discharge and to introduce the gas to be measured through an introducing pipe having a small conductance. With this complicated structure, there not only occurs a problem about cost and reliability, but there also occurs a problem in that the concentration of gas to be measured is reduced so that the sensitivity is deteriorated. Although in most cases, industrially speaking, the gas to be measured is at atmospheric pressure, differential air discharge has to be carried out through two or three stages. Thus, this is a serious problem.
Recently, an ultra small Q-pole type mass spectrometer which can be actuated in a high-pressure atmosphere of 0.1 to 1 Pa has been developed. Although, theoretically, this is the same as the ordinary Q-pole type mass spectrometer, the length of the Q-pole is shorter by about 10 mm ( 1/10 the ordinary type) so that mass separation is achieved in a shorter distance than the mean free path under 0.1 to 1 Pa. However, because the length of the Q-pole is short, the interval between the poles needs to be less than 1 mm and therefore the required positional accuracy of the Q-pole becomes very strict. Thus, currently, a sufficient performance cannot be achieved so that difficulty and cost of production increase.
On the other hand, the ordinary Q-pole type mass spectrometer has a serious fringing problem which deteriorates the sensitivity for a high-mass molecule. The fringing problem is generated because the electric field near an end face (fringing) of the Q-pole is weaker and disturbed more than near the center of the Q-pole. This is referred to as an end face electric field problem or end electric field problem. A specific ion, which is vibrated stably in the Q-pole having a normal electric field, turns to unstable traces and disperses in the fringing area, whereby the sensitivity is greatly reduced.
It has been known that while an influence of the entrance side (ion source side) of the Q-pole or entrance fringing region is very large, the exit side (collector side) or the exit fringing region has little influence. The reason is that mass separation is greatly affected if the injection direction and the position of an ion passing the entrance fringing region are deviated. But it is enough for the ion which passes the exit fringing region at least to enter the collector.
It is considered that the electric field is disturbed up to a distance equal to the pole interval outside and inside the Q-pole end face, so that it is considered that the fringing region becomes substantially twice the interval of the poles. Therefore, the Q-pole region in which the electric field is not disturbed is equal to a length obtained by subtracting the length of the fringing region from the length of the pole.
The influence of the fringing problem is increased proportionally to the vibration frequency in the fringing region. Thus, the degree of the bad influence is inversely proportional to the ion speed in the axial direction. That is, if the ion speed is slow, the time in which ion sojourns in the fringing region is prolonged, so that unstable vibration is repeated, thereby increasing the bad influence. It has been experimentally known that if the vibration in the fringing region is once or more, the bad influence is increased rapidly.
It is known that an ion having the same linear energy is slower if the mass thereof is increased, so that the fringing problem becomes very serious in the case of high mass. For example, if the length of the fringing region is 5 mm and the high-frequency vibration frequency is 2 MHz, when the ion speed is 5 eV, 15 eV and 30 eV, the vibration frequency in the fringing region is 0.5 times, 0.26 times and 0.2 times at 2 amu, 1.7 times, 0.98 times and 0.7 times at 28 amu, 2.3 times, 1.3 times and 0.9 times at 50 amu, 3.2 times, 1.7 times and 1 time at 100 amu, and 5.6 times, 3.2 times and 2.3 times at 300 amu. That is, the bad influence of the fringing problem appears at 28 amu or more at 5 eV and at 100 amu or more at 30 eV.
If the linear energy is increased, the sojourning time in the Q-pole is decreased so that the bad influence is reduced. However, the vibration frequency becomes short in the above mass separation so that a necessary resolution cannot be obtained. Thus, the linear energy of about 10 eV in which both the problems can be compromised is employed. However, under this condition, it has been known that the sensitivity drops to about ⅕ (one fifth) at 100 amu and about 1/100 (one hundredth) at 300 amu.
Conventionally, various methods have been considered as a countermeasure for the fringing problem. According to Japanese Patent Publication No. JP-B-40-17440, a Q-pole having plural segments each having different ratios between high-frequency voltage and DC voltage is employed. According to JP-B-40-17440, in the Q-pole on the injection side, by setting a large resolution, the fringing problem is reduced and by reducing the resolution successively, a required resolution can be obtained in a center pole. However, not only is the structure complicated, but there also occurs a new problem in that performance is deteriorated by a disturbance of the electric field between the Q-poles of respective segments. In JP-B-40-17440, although the Q-pole is divided into the respective segments, the potential on the axis of each Q-pole is the same. Therefore, the electric field in the axial direction on the axis is zero and the ion speed in the axial direction is constant.
According to Japanese Patent Publication No. JP-A-48-41791, a nozzle is disposed in the fringing portion. But there is a new problem JP-A-48-41791 in that the nozzle disturbs the electric field (data that it is ⅕ at 100 amu and 1/100 at 300 amu as the before described is a result of this nozzle system).
Instead of keeping the DC potential on the axis in the Q-pole at grounding potential, sometimes it is raised to 100 V while the potential of the ion source is kept at 110 V and the potential between the ion source and Q-pole is kept to 0 V. It is considered that an ion may pass the fringing region at a speed as high as 100 eV, so that a bad influence is reduced, and the ion may be decelerated in the Q-pole region and advance at a speed as low as 10 eV so that mass separation is carried out properly. This is the reason why the before mentioned combination of potential is adopted instead of keeping the DC potential on the axis in the Q-pole at grounding potential.
However, actually, this method has not produced any effect. There are two reasons for this. The first reason is that a large difference is generated between the potential on the axis in the Q-pole and the potential out of the Q-pole, so that the DC potential component is greatly disturbed whereby the bad influence of the fringing is further intensified. The second reason is that, correctly speaking, a position where the ion is decelerated is within the fringing region (near the ultimate end), but not within the Q-pole region. If the electric field exists in the axial direction the ion is decelerated and if the electric field in the axial direction is not completely uniform in a section vertical to the axis, a bad influence is produced. That is, that position is just the fringing region. Thus, under this method, the vibration frequency is not reduced within the fringing region. Particularly, this decelerating electric field is formed with symmetric electric fields comprised of the quadrupole electric field at the Q-pole and a uniform electric field (non electric field) outside of the Q-pole, so its section does not become uniform and the electric field is greatly disturbed.
Anyway, conventionally, there was no effective countermeasure for the fringing problem and there was not any Q-pole type mass spectrometer capable of measuring high-mass molecules at a high sensitivity.
In recent years, a three-dimensional quadrupole type mass spectrometer (named “ion trap”), which is similar to the Q-pole type mass spectrometer in its operation principle, has been developed for actual use. In an ion trap, ions are not mass-separated while traveling in a single direction like the Q-pole type mass spectrometer, but remain in the same region of the three-dimensional quadrupole for mass separation. However, the principle that only ions having specific mass/charge are detected by high-frequency electric field and DC electric field in the three-dimensional quadrupole is the same. These have been described in detail in Japanese Patent Publication No. JP-B-60-32310, Japanese Patent Publication No. JP-B-4-49219 and Japanese Patent Publication No. JP-B-8-21365.
In the ion trap, ions can be measured in a high-pressure atmosphere of 0.1 Pa because they do not have to be moved in the axial direction, and a high-mass gas can be measured without deterioration of the sensitivity because no fringing (end face) exists. Further, by a condensation function in which ions of a specific gas are accumulated and other ions are removed, an ultra small amount of gas can be measured.
But, in the ion trap, ions sojourn in the same region, so that a number of ions cannot be measured at the same time and its dynamic range is small owing to an influence of space charge. Further, in the ion trap, ion deposition and mass sweep are carried out alternately, so that complicated control is necessary and the ion source has no expandability.
In the ion trap, ions can be measured at a high pressure of 0.1 Pa. That is, even if an ion does not move in the axial direction, the ion is vibrated within the three-dimensional quadrupole, so that, practically speaking, a sufficiently long path exists in the ion trap. This path is longer than the mean free path under 0.1 Pa, and although the ion collides with the atmospheric gas many times there, mass separation is achieved without any problem. This indicates that even if the ion, which is vibrating in a stable condition, changes its path owing to collision with the atmospheric gas, it maintains stable vibration. This point is quite different from a magnetic field deflection type mass spectrometer in which, if an ion trace is changed halfway, a necessary initial condition is lost so that subsequent mass separation is completely impossible.
As a unit spectrometer using the same principle in use with a quadrupole electric field as the Q-pole type mass spectrometer, a quadrupole rail unit intended for non-contact holding and transportation of charged particles is available. There are some known methods. One is that the quadrupole electrode is tilted from a horizontal face so as to slide charged particles toward the center axis of the quadrupole, that is to say to slide charged particles in the axial direction downward by gravity. An other method is that an insulator charged with the same polarity as a particle is brought near the particle so as to move the particle in the axial direction by its reaction force.
In the method using gravity in the above described quadrupole rail unit, the mass of the particle (charged particle) is larger than a gas molecule. That is, the quadrupole rail unit is not intended for mass separation of a gas molecule, but is used for measurement of particles having a large mass or particles which are substance particles having a crystal structure. If this particle (charged particle) is an ionized gas molecule (ion), the time in which the ion passes the Q-pole region is extremely prolonged, so that it is never actually used as a measuring device.
In the method of using the reaction force against an insulator charged with the same polarity in the above described quadrupole rail unit, the charged particles injected into the Q-pole region must be pushed out of the Q-pole region by using a counter force of Coulomb force. As a result, the quadrupole electric field is inevitably disturbed so that proper mass separation is impossible. Further, according to this method, the driving mechanism is reciprocated along the Q-pole region so that the charged particle cannot be transported out of the Q-pole region. Therefore, this method is far from practical use as a mass spectrometer.
Although the quadrupole rail unit and the Q-pole type mass spectrometer use the same principle of using the quadrupole electric field, the quadrupole rail intended for mainly non-contact holding and transportation of large particles for investigation and the Q-pole type mass spectrometer intended for mainly continuous mass separation for measurement are completely different from each other in terms of application, function and structure. Therefore, the application of the particle transportation method by the quadrupole rail to the Q-pole type mass spectrometer is completely impossible from the viewpoints of performance and practical use.
Under a high-pressure atmosphere of more than 0.1 Pa in the conventional Q-pole type mass spectrometer, an ion collides with the atmospheric gas so that the speed in the axial direction is reduced to zero, so that it is stopped in the Q-pole region and thus is not detected by a collector.
There is a known method of using gravity force or a counter force by bringing an insulator charged with the same polarity as that of particle to be measured in the quadrupole electric field for transporting charged particles. But continuous mass separation for gas molecules can not be carried out by the known method.
If an ion is injected at high speed in order to reduce the influence of an end electric field near a Q-pole end face (fringing) which deteriorates the sensitivity of the Q-pole type mass spectrometer, the ion passes the Q-pole region at high speed, so that the necessary vibration frequency cannot be obtained and proper mass separation is not carried out.
Further, there is also a problem in that a condensation function cannot be carried out so that an ultra-small amount of gas cannot be measured.